"This has been a very dangerous Islamic region - not just Chechnya, but the north Caucus countries. Right now, the big question will be not just who were their local collaborators, but to what extent were they encouraged or supported from foreign terrorist groups."

"My assumption is that these people were alienated by their non-assimilation in this culture but probably were linked in some way, either stealth-radicalization over the Internet or directly encouraged and shaped by foreign terrorist organizations."

"I think there's a question of where the explosives came from. Everyone should remember the nature of terrorism - they want the highest profile they can obtain, they are out making a statement, they are creating fear. It's the maximum impact. You have the equivalent of a dead man walking right now. I think the police and the FBI need the information from the people that are inside and around that perimeter. Anything unusual, call it in right away."

"As long as they're on the move, they're going to make a mistake, they're going to do something that is observable, that is detectable and should be immediately reported."

"(Authorities) are pretty close. I imagine this will be resolved in the next 24 hours at maximum. The only remaining question is are there other assailants out there, other people who were part of the support network who helped them plan this. Were there friends and family who maybe knew about this but didn't want to bring it to authorities and to some degree they played a passive role. And that's one of the things that authorities are going to be trying to find out."

"I suspect that this is going to end in death. This individual has lost his brother, he's come to this country to commit an act of terror. He didn't leave the scene—that is Boston—when he could. My guess is that he's either going to die from suicide or he's going to be killed by police."

"This looks to me like a closed cell. We haven't seen more than two brothers. Only two people appear every time. One is 26 one is 19. That tells me that you probably had a radicalizer here, that's the older brother, who persuaded the younger brother to do this. They don't seem to have engaged, at least at this point, to have involved another operator."

"[The] lack of sophistication of the device does suggest that this is what I'd call the 'B' team. This is not the 'A' team. And if you look at other signatures here, the way they actually executed, the explosions. They were ... done within 15 seconds of each other as opposed to doing a second sequence where you blow the first device—wait for the first responders to come on scene—to get second wave of casualties with a second device."

"The thing that makes me think it's ideological, in part, is the target. Like 9/11, when the targets—if it hadn't been for the heroes on Flight 93—would have included the Congress, the Pentagon, and the World Trade Center. These are emblematic of the United States. Patriots Day in Boston commemorates the American Revolution."

"It is something of an ideological enemy not just someone who was crazed for workplace violence on something like that. … When you target something that is symbolic of the United States, you probably think—or the group you're part of thinks of itself—as an enemy of the United States in some way."

"In terrorist attacks overseas, kids are trained at a fairly early age. ... They've identified these two gentlemen coming from Chechnya, which has been a war zone forever. The likelihood that they've received some military or paramilitary training is probably relatively high, especially given their ability to fashion different improvised explosive devices."

6:19 a.m. ET — Roger Cressey, former National Security Council official in the Clinton and Bush administrations and current NBC News consultant:

"It's remarkable development that it happened so quickly. If this was the result of the news conference yesterday where the FBI publicized both photos, we see the benefit of that."

"The FBI won't have released the pictures if they had other leads. The whole purpose of doing it was to get the public involved. If anyone is second-guessing right now, "A" they're foolish and "B" if the reason these individuals are caught it's because of the action yesterday."

"It's possible they did it themselves. But my thought is there is other people involved from a direction standpoint, an organization standpoint. ... Sounds to me like this is a bigger thing. But we don't know that yet. The police and the FBI are going to be working hard over the next several days."

"[Authorities] are going to have the names and they can certainly trace back the history of when they came into the country. Were they students? Did they just get here? There are going to be paper records, electronic records of everything that lead up to them coming to the United States. So they'll get that all figured out in time."